The invention relates to a method in a soda recovery boiler in which flue gases are led through a so-called economizer to recover heat from flue gases. The invention also relates to a soda recovery boiler comprising a furnace and an economizer which is arranged in the flow of flue gases to recover heat from flue gases exiting the furnace.
In chemical pulping industry, soda recovery boilers are used not only for the recovery of chemicals but also for the production of energy. As to the general operating principle and structure of soda recovery boilers, reference is made, for example, to European patent 737260 and U.S. Pat. No. 6,178,924.
The soda recovery boiler comprises a furnace, a system for feeding boiler supply water, a superheater at the upper part of the furnace, possibly a boiler bank (array of boiler tubes), and, after these in the flowing direction of flue gases, a so-called economizer for the recovery of thermal energy contained by in the flue gases. The soda recovery boiler also comprises a combustion air supply for introducing the required combustion air in the furnace. Leading the supply water through different parts of the boiler produces high-pressure steam which is at a high temperature and can be used for the production of electricity with a steam turbine.
The aim is to utilize the heat contained in the flue gases in the economizer, in which it is used for heating the supply water before it is passed to steam production, as described for example in U.S. Pat. No. 5,769,156.
In soda recovery boilers, it is also known to cool the flue gases with a so-called circulation water cooler for flue gases, if the supply water is too hot for bringing the flue gases to a sufficiently low temperature, the circulation water cooler for flue gases being connected to the supply water flow circuit in the soda recovery boiler. The supply water is normally heated in a supply water tank by means of bleed steam extracted from a steam turbine. In soda recovery boilers, the temperature of the supply water tank must often be reduced by throttling the steam entering it, to make the supply water sufficiently cold to cool the flue gases. At present, heat exchange systems in soda recovery boilers do not take into account the efficiency in view of the production of electricity. The throttling of steam and the introduction of heat in the supply water at a cold temperature is not advantageous for the, yield of electricity from the steam process.
It is an aim of the invention to present a method in a soda recovery boiler to improve the efficiency of the production of electricity. It is another aim of the invention to present an improved soda recovery boiler for the above-mentioned purpose.
In the method according to the invention, the final cooling of the flue gases is performed by a circulation water cooler, separately from the supply water system. Consequently, the flue gases are not cooled entirely with supply water. The circulation water cooler is used to introduce the heat of the flue gases to the combustion air instead of the supply water. Pre-heating of the supply water is carried out with flue gases before said circulation water cooler, seen in their flowing direction; that is, the flue gases are cooled only in part with supply water, at the stage where they are initially at a higher temperature.
The higher the average temperature at which the heat is introduced from the flue gases of the soda recovery boiler to the supply water, the better is the yield of electricity. Consequently, it is advantageous to cool the flue gases with supply water until their last cooling stage which is accomplished with the circulation water cooler. The supply water used for cooling the flue gases is preferably preheated with high-pressure steam originating in the steam production of the same boiler, for example, with bleed steam and/or back-pressure steam of a steam turbine. The heat recovered by the circulation water cooler in the last cooling stage of the flue gases can be used to heat the combustion air to a high temperature, and it can be heated further with high-pressure steam.
In the economizer of the soda recovery boiler according to the invention, there is, in the last stage, a circulation water economizer connected to the circulation water cooler of the flue gases, where the water is circulated through a heat exchanger, that is in a heat transfer connection with a combustion air supply channel, and in the supply water economizer stage preceding said circulation water economizer, there is a heat transfer arrangement for the transfer of heat from the flue gases to the supply water.